Heres my 20 minute routine...
First 2 minutes - Warmup at 50% of max intensity
3rd minute - 60% intensity
4th minute - 70% intensity
5th minute - 80% intensity
6th minute - 90% intensity
7th minute - 60% intensity
8th minute - 70% intensity
9th minute - 80% intensity
10th minute - 90% intensity
11th minute - 60% intensity
12th minute - 70% intensity
13th minute - 80% intensity
14th minute - 90% intensity
15th minute - 60% intensity
16th minute - 70% intensity
17th minute - 80% intensity
18th minute - 90% intensity
19th minute - 100% intensity
20th minute - 50% cool down intensity
Now, I ride a
stationary bike for this, and my bike has resistance levels that i can increase , or i can watch the LCD and it shows my MPH (speed) that im pedalling. Sometimes i increase both speed and resistance, or just one of them.
You can do this on any kind of cardio you want. Even a walk. You can walk fast and then walk up a hill or something for the more intense parts. Its very effective. Im seeing great results.
This also forces your heart to recover while still under stress, which is very good. The best way would probably be to alternate or change it up every now and then. Do the
HIIT cardio for a week, then the next do the 45 min constant intensity cardio, or something liek that. I just like the fact that i can be done in 20 mins and still get a very good cardio workout, because i HATE cardio.